If you know an agent represents an author whose work is similar to yours (but not so close as to be in direct competition) do you mention this in the query or not? Our works are similar in genre, but quite different in content. But is it good or bad to mention it or will it not make the slightest difference? This agent seems a good fit in every way, is open to queries and I want to query him in the new year.

AFAIK it is ok to mention other authors in terms of identifying your target audience. "This book will appeal to fans of Jim Butcher and Kim Harrison".

However, it is not ok to compare the book to other authors' books. "This book is like Harry Potter, but better!" "It's just like The Davinci Code!" Or "This book is the next Twilight!" And you might not want to offer a lot of authors with different audiences. "This book will appeal to fans of J.K Rowling, Stephen King, Terry Pratchett and Nora Roberts!"

That said, I don't think it would make a big difference. The body of the query is more important, imo, than being able to identify the audience for the agent--who probably has a better idea of who would want to read your book than you do :)